There exist a wide variety of industries and further a wide variety of applications within such industries in which cutting, sealing and/or patterning of plastic materials is required. For example, in the inflatable toy field, it is conventional to provide means for sealing marginal edges of plastic sheets to one another along a seam of a predetermined contour and to cut and tear away the unneeded portions of the joined sheets immediately adjacent the seal (commonly referred to as a tear-seal operation). Other types of operations are utilized in which plastic materials and the like are joined to one another as well as being joined to a fabric substrate of either the woven or non-woven type. Still further applications call for the lamination of the plastic material to metal or the patterning of a plastic material with or without lamination and cutting. In all such applications it is quite frequently advantageous to employ dielectric heating for the heat-sealing of plastic films and fabrics. In such applications, radio frequency (RF) energy is introduced into the work area in the form of a dielectric field produced by a source of high frequency energy having a magnitude of voltage suitable for the thickness of the particular work load. The types of loads typically encountered possess a loss characteristic which causes a rise in temperature in the work pieces produced by the electric field.
The use of dielectric heating equipment for the bonding of thermal plastic materials and the like is a conventional technique. Typically, thin sheets of the thermal plastic material are sandwiched under at least moderate pressure between a pair of metallic electrodes to form a capacitance. A radio frequency high power oscillator is utilized to apply high frequency energy across the capacitance during a predetermined time interval. Heat is generated throughout the thermal plastic material due to the fact that it has a considerable power factor. The heat is generally readily conducted away by the electrodes at the outer surfaces of the two sheets, but not at the inner face thereof. Thus, the temperature at the interface rises to the melting point of the plastic material causing fusing of the two sheets to take place.
In applications in which two plastic members are to be joined, it is conventional to utilize die assemblies of either the compound or fixed die types. Fixed die assemblies consist of a die face and a rigid member having a tapered edge positioned immediately adjacent thereto. The plastic members to be joined are aligned beneath the die face and pressure is exerted upon the plastic members, at which time RF energy is applied. The tapered edge pierces nearly through the sheet (or sheets) and the excess may be stripped away either during the heat-sealing operation or at some time thereafter. One of the major problems in the fixed die assembly results from the fixed relationship between the height of the face of the tapered edge and the face of the sealing or patterning die elements. Once a die is fabricated it becomes limited in its usefullness in terms of a wide range of thickness of plastic and fabric loads that differ in thickness radically from the design range of the die. This would require the employment of a large number of die assemblies to accommodate the entire range of thicknesses of the possible loads. Upon completion of the sealing and tearing operation a marginal portion of at least one of the plastic sheets remains and extends beyond the edge of the seal and in between the seal and the tapered edge, if they are spaced apart. This may result in a fraying or deterioration of the edge. Fraying may occur especially in the case where one or more of the plastic sheets is secured to a fabric substrate.
In die assemblies of the compound type, the cutting edge is positioned adjacent to the die face but, contrary to the arrangement in the fixed die assemblies, the cutter is movable relative to the die face. The sealing and cutting operation is performed in two-step fashion wherein a first magnitude of pressure (usually moderate pressure) is applied to cause the die face to be urged under pressure, in the presence of RF energy, against the plastic sheet which may be reinforced with additional plastic sheets or a substrate or interstitial layer of a fabric, metal or other non plastic material not affected by the RF energy so as to melt or directly heat and cause to flow the material comprising that non-plastic layer. Thus, the layer must be cut by conventional means. Upon completion of the heat-seal operation, additional pressure is applied to urge cutter into the plastic sheet (or load) and thereby perform the cutting operation. The major disadvantage of conventional compound die assemblies also results in the fact that a finite separation exists between the cutting edge and the die face, causing a marginal portion of the plastic sheet (or load) to be present after the cutting operation which may further result in a fraying of the material or in a weakening of the seal, as well as providing an unsightly finish to the members being joined.
Another conventional heat-sealing operation employing RF techniques is that in which a roller is employed to form a pattern or texture upon the surface of a continuously fed plastic sheet (or sheets). One typical embodiment for embossing and/or joining plastic sheets is described in detail in U.S. Pat. No. 3,162,561, issued Dec. 22, 1964 and filed in the name of the present inventor. Although the arrangement of the aforementioned U.S. Patent will not be described in detail, it should be understood that a cylindrical roller having a metallic die face serves as one of the electrodes of the RF heat-sealing apparatus with a continuous closed loop belt serving as the other electrode. In applications wherein the two or more sheets are to be continuously sealed to one another, the drum may have a perfectly plain cylindrical surface. In cases wherein raised or depressed printing is to be provided, the drum may, in turn, be provided with a depressed or raised surface, respectively. In cases where a texture or pattern is to be provided over substantially the entire surface, the surface of the drum is provided with a metallic sheet which is engraved by hand to provide the particular texture or pattern desired. In assemblies of the fixed or compound die type, as well as assemblies of the continuous sealing and/or embossing type, the die faces are typically formed by hand to provide the desired texture since any other technique will not provide the shading of texture required in many applications. Hand-tooled operations of this type require a great deal of time and are therefore quite expensive.